The resin transfer molding (RTM) technology has been widely used in many fields. In the RTM process, permeability plays an important role in model building, process control and simulation. Permeability denotes the ability of a fiber material to transport a fluid and functions as an index to evaluate the behavior of filling resin. Therefore, accurate measurement of permeability will favor process simulation and process control greatly and promote the yield of the products of a fiber reinforced polymer (FRP) composite material.
So far, there have been many researches studying the permeability of pre-woven fiber objects (preforms). For example, Lee et al. (Y. J. Lee, J. H. Wu, Y. Hsu, and C. H. Chung) proposed a paper “A prediction method on in-plane permeability of mat/roving fibers laminates in vacuum assisted resin transfer molding,” Polymer Composites, vol. 27, pp. 665-670, 2006, which derived an equation according to the Darcy's Law, the continuous equation of incompressible fluid, and the in-plane permeabilities of a single type or several types of fiber planes, and defined the thickness-wise porosity to establish a prediction method. The prediction method finally generated a group of equations for predicting the in-plane permeabilities at the top and bottom surfaces of mat preform. The equations exempt the manufacturer from spending much time in permeability measurements.
The prior arts all supposed that the permeability or filling pressure of the preform is a constant. Thus, the acquired permeability is an average permeability. If the manufacturer wants to improve the quality of products, on-line permeability measurement and local permeability measurement are necessary.
The local permeability measurement methods include the Gas-Assisted Real-time Assessment of Permeability (GRASP) proposed by Ding et al. (Li Ding, Chiang Shih, Zhiyong Liang, Chuck Zhang, and Ben Wang) in a paper “In situ measurement and monitoring of whole-field permeability profile of fiber preform for liquid composite molding processes” in Composites Part A: Applied Science and Manufacturing, vol. 34, pp. 779-789, 2003; the fuzzy logic model proposed by Pitchumani et al. (D. R. NIELSEN and R. PITCHUMANI) in a paper “Control of Flow in Resin Transfer Molding With Real-Time Preform Permeability Estimation” in POLYMER COMPOSITES, vol. 23, pp. 1087-1110, 2002; and the simulation database proposed by Devillard et al. (MATHIEU DEVILLARD, KUANG-TING HSIAO, ALI GOKCE AND SURESH G. ADVAIN) in a paper “On-line Characterization of Bulk Permeability and Race-tracking During the Filling Stage in Resin Transfer Molding Process” in Journal of COMPOSITE MATERIALS, vol. 37, pp. 1525-1541, 2003. However, incompleteness of the historical data impairs the validness of the fuzzy logic model and the simulation database.
In the practical RTM process, the non-homogeneous preform, the irregular fiber distribution caused by incorrect operation, or the overused spray glue may result in uneven distribution of permeability, causing the resin to flow toward the low-resistance direction. Thus, the low-permeability regions not completely filled with resin may lead to defects of misruns or dry spots. In order to overcome the problem, the operators manually adjust the process variables, such as the injection pressure, to improve resin flow and the product quality. However, the decisions of the variables lack theoretical groundwork but all rely on the operators' experience.